We add an option not to enable fbdev, this option is off (0) by default.
Not enabling fbdev at load time makes it possible to co-operate with
vga16fb and vga text mode when VT switching.

However, if 3D resources are active when VT switching, we're currently
not able to switch over to vga, due to device limitations.
This fixes a bug where we previously lost 3D state during VT switch.
Signed-off-by: NThomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>